1. Field of the Invention
The present invention relates to a rotation angle detecting device for detecting a rotation angle of a rotating object such as a crank shaft of an engine or a wheel of a vehicle.
2. Description of the Related Art
JP-2003-75108-A discloses a rotation angle detecting device. In such a rotation angle detecting device, a change in magnetic field caused by rotation of a permanent magnet rotor is detected by a pair of magnetic sensor elements for providing a pair of output signals. The output signals are converted into trigonometric functions to calculate the rotation angle of the rotating object.
Such a rotation angle detecting device has a difficulty in reducing its size because the pair of sensor elements is disposed distant from each other along the circumference of a permanent magnet rotor.
Further, it is difficult to accurately position the sensor elements around the permanent magnet rotor because of vibration when the rotation angle detecting device is installed into a motor vehicle or the like. As shown in FIG. 18, the magnetic flux density distribution around a permanent magnet rotor 400 is not uniform. Accordingly, one of the sensor elements may sense a change in the magnetic flux density that is different from the other sensor element if the sensor elements are not accurately positioned. Further, because such a sensor element changes its output signal as the temperature surrounding the sensor changes, the magnetic sensor may provide an inaccurate output signal if the temperature surrounding one of the sensor elements is different from the temperature surrounding the other sensor element.
If a pair of magnetic sensor elements 412, 412′ is integrated into a unit 410 that is disposed at a position around the permanent magnet rotor 400 as shown in FIG. 16, the size of the rotation angle detecting device can be reduced. In this case, the first sensor elements 412 is disposed so that the first sensor surface 413 thereof can be perpendicular to a line intersecting the rotation axis 404 of the permanent magnet rotor 400 at right angles and the second sensor element 412′ is disposed so that the second sensor surface 413′ thereof can be perpendicular to the first sensor surface 413.
However, the sensor elements 412, 412′ provide output signals whose amplitude are different from each other as shown in FIG. 17 when the permanent magnet rotor 400 rotates, because the sensor elements 412, 412′ are respectively disposed in the magnetic field of different magnetic flux density. Therefore, it is difficult to accurately convert the output signals to trigonometric functions.